1. Field of the Invention
The present invention relates to a television receiver and, more particularly, to a contrast correction device which can be used to correct the contrast in a video signal in a television receiver, a video cassette recorder, or the like.
2. Description of the Prior Art
As the screen size of color television receivers has increased in recent years, contrast correction devices have become increasingly important as a means of expanding the dynamic range of the video signal reproduced on the CRT screen by correcting the contrast of the video signal for a sharper image by passing the video signal through a non-linear amplifier such as a gamma correction device.
One type of such device is a described below with reference to the accompanying figures.
FIGS. 6, 7a and 7b are, respectively, a block diagram and graphs showing the input/output relationship in a first example of a conventional contrast correction device. As shown in FIG. 6, the contrast correction device comprises a gamma correction circuit 1 which corrects the luminance signal S1 input thereto so as to suppress white or bright tones according to an average detected luminance signal S3'; an average luminance detection circuit 2 which averages the luminance signal S2' over one or a plurality of frames and produces an average detected luminance signal S3'; and a matrix circuit 3 which adds the luminance signal S2' and input color difference signal S4 to output the color signal S5' to CRT 4.
A contrast correction device thus constructed operates as described below.
First, the post-correction luminance signal S2' is input to the average luminance detection circuit 2 for averaging the signal S2' over a predetermined time which may be more than one vertical scan period to detect the average luminance signal S3'. The detected average luminance signal S3' is input to the gamma correction circuit 1. The gamma correction circuit 1 changes the input-output characteristics according to the average detected luminance signal S3' as shown in FIGS. 7a and 7b.
More specifically, when the average detected luminance signal S3' is of dark, white or light tones are suppressed and black tones are enhanced as shown in FIG. 7a. On the contrary, when the average detected luminance signal S3' is bright the suppression of white or light tones is weakened as shown in FIG. 7b so that white or light tones are enhanced more than that which resulted from the case shown in FIG. 7a. The thus obtained post-correction luminance signal S2' is input to the matrix circuit 3 to be added to the input color difference signal S4, resulting in the generation of the color signal S5' used to drive the CRT 4 to generate the video image.
FIGS. 8, 9a and 9b are, respectively, a block diagram and graphs showing the input-output characteristics according to a second example of a conventional contrast correction device. When compared with the example shown in FIG. 6, the contrast correction device shown in FIG. 8 has, in place of the average luminance detection circuit 2, a white peak level detection circuit 6 which detects the whitest level of the post-correction luminance signal S2' to output a white peak signal S6', and a white peak comparison circuit 7 which compares the white peak detection signal S6' with a predetermined white peak level signal Vg' set externally and outputs the result as a white peak comparison signal S8'.
The contrast correction device of FIG. 8 operates as described below.
First, the post-correction luminance signal S2' is input to the white peak level detection circuit 6 to detect the white peak level (the luminance of the whitest part of the video signal) over a predetermined time which may be more than one vertical scan period. The detected white peak level is output as the white peak detection signal S6' (the level of which increases as the white peak level increases) and is compared by the white peak comparison circuit 7 with an externally set white peak level voltage Vg', thus yielding the white peak comparison signal S8', which is input to the gamma correction circuit 1.
The gamma correction circuit 1 changes the input-output characteristics as shown in FIGS. 9a and 9b according to the white peak comparison signal S8'. When the voltage level of the white peak level voltage Vg' is equal to or greater than the white peak detection signal S6' (i.e., when the image is dark overall), the light tones are suppressed as shown in FIG. 9a to enhance the dark tones. When the white peak level voltage Vg' is less than the white peak detection signal S6' (i.e., when the image is bright overall), correction is suppressed as shown in FIG. 8b for better enhancement of the light tones when compared with FIG. 9a. The post-correction luminance signal S2' thus obtained is input to the matrix circuit 3 where it is added to the input color difference signal d to generate the color signal S5', which is then used to drive the CRT 4 and generate the video image.
However, with conventional contrast correction devices as described above, when the average luminance is bright in the first example, or the white peak detection signal S6' level is high (bright) in the second example, the gamma input-output characteristics of light or white tones are simply made approximately linear. This does not make it possible to expand the dynamic range, resulting in the need to further expand the dynamic range to achieve a high image quality in large-screen television displays.